The removal of lignin in lignocellulosic materials such as chemical pulp is accomplished by a multi-stage application of bleaching chemicals. Chlorine dioxide is the chemical of choice because it reacts readily and selectively with lignin and does not react to any significant extent with carbohydrate. A typical bleaching sequence uses a chlorine dioxide delignification stage (notated as D0), an alkaline extraction stage in which oxygen gas and peroxide are commonly added (notated as Eop), and a final brightening procedure which can comprise one chlorine dioxide stage (notated as D) or two chlorine dioxide stages (notated as D1 and D2, respectively) with or without an intermediate extraction stage.
A final chlorine dioxide bleaching stage, such as D or D2, is typically run at a temperature between 55 to 85° C. with a retention time between 2 and 4 hours and a consistency between 8 and 15%. The pH is typically adjusted with sodium hydroxide or sulphuric acid before the chlorine dioxide to provide a final pH of between 3.5 and 4.0.
Prior art shows that the maximum brightness development is achieved by keeping the pH during the final brightening stage very close to neutral (5.0 to 7.0). It has been demonstrated in the laboratory that, by using soluble buffers such as potassium dihydrogen phosphate, maximum brightness is achieved at a pH between 5.0 and 6.5 [1]. Canadian Patent No. 756,967 discloses a process for neutral chlorine dioxide bleaching wherein neutral conditions are maintained by the addition of carbonates, oxides of alkaline earth materials, or bicarbonates of alkali or alkaline earth metals which are of sufficiently limited solubility [2]. Neither of these processes are practised commercially. The soluble buffers such as potassium dihydrogen phosphate are too expensive for industrial application while the handling of sparingly soluble buffers such as sodium bicarbonate is difficult. For these reasons present industrial practice is to adjust pH at the beginning of the stage with either a soluble alkali or acid to provide a final pH of between 3.5 and 4.0, which has been reported as the optimal end pH under unbuffered conditions [3,4].